‘Probing the Earth with isotopes’ shows how, using isotopes, we have come to understand the structure and behaviour of the Earth. The outer few tens of kilometres are divided into continental and oceanic crust. Below the crust, the sub-surface is divided into the mantle and the core. From the base of the crust to about 2,800 km depth, the Earth is rocky and composed of minerals like olivine and pyroxene that are rich in magnesium, iron, and calcium. From about 2,800 km to about 5,100 km depth the outer core is liquid. The remaining 1,200 km or so to the centre of the Earth is solid metal.